Conventional flat-panel displays made in accordance with known liquid crystal display technologies have heretofore been both limited in size and expensive to manufacture. A large display device may be constructed at reduced cost by assembling multiple smaller display "tiles". However, it is necessary to make the internal seams visually imperceptible to create a pleasing display.
For the seams to be visually imperceptible and for the display image to be sharp, the light used to illuminate the display must be highly collimated. A collimated light source must allow essentially no visible energy to radiate beyond an allowable off-normal angle. The allowable off-normal angle is prescribed by the tile thickness and the cover plate mask and back plate mask dimensions. It is defined as the critical off-normal angle below which light from the illumination source must not enter the tile to tile seam area. This type of tiled display construction is described in U.S. Pat. No. 5,661,531, entitled "Construction and Sealing of Tiled, Flat-Panel Displays"; and co-pending U.S. patent application, Ser. No. 08/593,759, filed on Jan. 29, 1996, entitled "Tiled, Flat-Panel Display Having Invisible Seams". Both U.S. Pat. No. 5,661,531 and co-pending application Ser. No. 08/593,759 are hereby incorporated by reference.
Typical practice for LCD illumination uses area light sources such as fluorescent tube arrays. A collimator must focus the light from the light source forward, toward the flat-panel display, forcing essentially all visible light energy to fall within the off-normal angle described hereinabove. Most commonly used collimators do reduce the light intensity at large off-normal angles, but do not perform well enough at small off-normal angles for use with a tiled, flat-panel display having visually imperceptible seams. A seamless appearance in a tiled, flat-panel display requires that unwanted visible light energy outside of the off-normal angle be reduced to less than one percent of the intensity of the light at a normal angle. This percentage is derived in a 1992 reference paper by G. Alphonse and J. Lubin entitled "Psychophysical Requirements for a Tiled Large Screen Display" published in SPIE Journal, Volume 1664, pp. 230-240.
In tiled, flat-panel constructions featuring a cover plate with an integral screen, the light must also be collimated to such an extent that essentially no light from one pixel can reach the screen area associated with any other pixel. Adherence to this requirement produces the sharpest possible image on the tiled, flat-panel display.
It is therefore an object of the invention to provide an apparatus and method for producing highly collimated light suitable for use with a tiled, flat-panel display having visually imperceptible seams.
It is a further object of the invention to provide a means of reducing the intensity of visible light energy, which falls outside of a desired, off-normal angle, to an acceptable level.
It is yet another object of this invention to maximize the pixel resolution in tiled, flat-panel displays by providing highly collimated light.
It is a further object of this invention to produce a wide area, collimated light source having a small depth to enable building tiled, flat-panel displays having a small overall thickness.
The present invention provides an apparatus and method for producing the highly collimated light required for use with a seamless, tiled, flat-panel display.